Negative electrode plate, preparation method thereof and electrochemical device

ABSTRACT

The invention refers to negative electrode plate, preparation method thereof and electrochemical device. The negative electrode plate comprises: a negative current collector; a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer comprising opposite first surface and second surface, wherein the first surface is disposed away from the negative current collector; and an inorganic dielectric layer consisting of an inorganic dielectric material disposed on the first surface of the negative active material layer. The negative electrode plate provided by the application is useful in an electrochemical device, and can result in an electrochemical device having simultaneously excellent safety performance and cycle performance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Chinese Patent Application No.201810712570.1, filed on Jun. 29, 2018, which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

The invention belongs to the technical field of energy storage devices,particularly refers to a negative electrode plate, a preparation methodthereof and an electrochemical device.

BACKGROUND

Electrochemical device, which can convert chemical energy into electricenergy, has advantages including stable voltage and current, reliableperformance, simple structure and being easy to carry. It has become themain power source for various consumer electronic products and electricproducts, and is widely used in all aspects of production and life ofmankind. In recent years, the market has placed increasing demands onthe cycle performance and safety performance of electrochemical devices.

However, in the existing electrochemical devices, there are many sidereactions between electrolyte and negative electrode interface duringcycle process, which cause decomposition of electrolyte and gasproduction, thereby deteriorating safety performance of electrochemicaldevices and continuously consuming electrolyte cation on negativeelectrode. Irreversible capacity of the electrochemical device iscontinuously increased, resulting in rapid decay of cycle life.Moreover, the poor transmission performance of electrolyte cation innegative electrode also causes the formation of metal dendrites on thenegative electrode interface, which easily induces short circuit in theelectrochemical device and further deteriorates safety performance ofelectrochemical device.

SUMMARY

The application provides a negative electrode plate, a preparationmethod thereof and an electrochemical device, to improve safetyperformance and cycle performance of electrochemical device.

The first aspect of the application provides a negative electrode plate,comprising: a negative current collector; a negative active materiallayer disposed on at least one surface of the negative currentcollector, the negative active material layer comprising opposite firstsurface and second surface, wherein the first surface is disposed awayfrom the negative current collector; and an inorganic dielectric layerconsisting of an inorganic dielectric material disposed on the firstsurface of the negative active material layer, wherein inorganicdielectric layer comprises a first dielectric layer on an outer surfaceof the negative active material layer, and the first dielectric layerhas a thickness of from 30 nm to 1000 nm and has a crack extendingthrough the thickness of the first dielectric layer.

The second aspect of the application provides a method for preparationof a negative electrode plate comprising the steps of: disposing anegative active material layer on at least one surface of a negativecurrent collector; depositing an inorganic dielectric material on asurface of the negative active material layer facing away from thenegative current collector by vapor deposition, to obtain an initialinorganic dielectric layer including an initial first dielectric layeron an outer surface of the negative active material layer; applyingpressure to the initial first dielectric layer to form a crack extendingthrough the thickness of the initial first dielectric layer to obtain afirst dielectric layer as an inorganic dielectric layer, wherein thefirst dielectric layer has a thickness of from 30 nm to 1000 nm.

The third aspect of the application provides an electrochemical device,comprising a positive electrode plate, a negative electrode plate, aseparator and a electrolyte, wherein the negative electrode plate is thenegative electrode plate according to the first aspect of theapplication.

As compared with the existing prior art, the application has at leastthe following beneficial effects:

Due to the inorganic dielectric layer disposed between negative activematerial layer and separator and the formation of crack in the inorganicdielectric layer, the negative electrode plate of the application canstabilize negative electrode interface, and reduce side reactionsbetween electrolyte and negative electrode interface, inhibit gasproduction, avoid an increase in irreversible capacity. The negativeelectrode plate of the application can also increase wetting andretention of electrolyte on negative electrode plate as well as ionpermeability, improve the kinetic performance of the electrochemicaldevice, inhibit the precipitation of the reduced metal of electrolytecation on the negative electrode, and avoid short circuit inelectrochemical device. Therefore, the electrochemical device hassimultaneously excellent safety performance and cycle performance, andthe improved high temperature performance.

In addition, the negative electrode plate of the application cansignificantly inhibit the increase of Direct Current Resistance (DCR)during cycle and storage process of electrochemical device, therebyreducing thermal effects, reducing polarization, and improving rateperformance.

DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions in theembodiments of the application, the figures used in the embodiments ofthe application will be briefly described below. A person skilled in theart can further obtained other drawings from these figures, without anycreative work.

FIG. 1 shows a schematic view of the structure of a negative electrodeplate provided by an embodiment of the application.

FIG. 2 shows a schematic view of the structure of a negative electrodeplate provided by another embodiment of the application.

FIG. 3 shows a schematic view of cracks.

FIG. 4 shows a schematic view of the structure of a negative electrodeplate provided by yet another embodiment of the application.

FIG. 5 is an enlarged view of area I of FIG. 4.

FIG. 6 shows a schematic view of the structure of a negative electrodeplate provided by still another embodiment of the application.

FIG. 7 is an enlarged view of area II of FIG. 6.

DETAILED DESCRIPTION

In order to make the object, technical solutions and beneficialtechnical effects of the present disclosure more apparent, hereinafterin combination with examples, the present disclosure is furtherdescribed in detail. It should be understood that examples in thepresent disclosure are only to explain the present disclosure, and arenot intended to limit the present disclosure.

For the sake of brevity, only certain ranges are explicitly disclosedherein. However, ranges from any lower limit may be combined with anyupper limit to recite a range not explicitly recited; ranges from anylower limit may be combined with any other lower limit to recite a rangenot explicitly recited; and in the same way, ranges from any upper limitmay be combined with any other upper limit to recite a range notexplicitly recited. Additionally, within a range includes every point orindividual value between its end points even though not explicitlyrecited. Thus, every point or individual value may serve as its ownlower or upper limit combined with any other point or individual valueor any other lower or upper limit, to recite a range not explicitlyrecited.

In the description herein, it is to be noted that unless otherwisestated, the words “above” and “below” are inclusive of the case where itis equal to, and the word “more” in fragment “one or more” means two ormore.

The contents of the above disclosure are not intended to describe eachand every example or embodiment disclosed herein. More exemplaryembodiments will be described below in more details by way of examples.At a plurality of places throughout the present disclosure, a series ofexamples are provided to give teaching, and these examples can becombined in any way, if possible. In each example, the exemplificationis just for illustrative purpose, and shall not be interpreted asenumeration.

Negative Electrode Plate

Referring to FIGS. 1 and 2, the first aspect of an embodiment of theapplication provides a negative electrode plate 10, comprising anegative current collector 11, a negative active material layer 12 andan inorganic dielectric layer 13 which are provided in a stacked manner.

The negative current collector 11 includes two opposite surfaces alongits thickness direction. The negative active material layer 12 may bedisposed on either one of the two surfaces (as shown in FIG. 1) or maybe disposed on both of the two surfaces respectively (as shown in FIG.2). The negative active material layer 12 includes opposite firstsurface 121 and second surface 122 along the thickness directionthereof, wherein the first surface 121 is disposed away from thenegative current collector 11, and the second surface 122 is disposedtoward the negative current collector 11. The negative active materiallayer 12 comprises negative active material, and has a large number ofpores 123 therein. Reversible de-intercalation/intercalation ofelectrolyte cations may be carried out during operation. The negativecurrent collector 11 is used for current collection and output.

The inorganic dielectric layer 13 is disposed on the first surface 121of the negative active material layer 12 and consists of an inorganicdielectric material. The inorganic dielectric layer 13 comprises firstdielectric layer 131 outside the negative active material layer 12. Thefirst dielectric layer 131 has a thickness T of from 30 nm to 1000 nm.Further, referring to FIG. 3, the first dielectric layer 131 has crack1311. Specifically, the first dielectric layer 131 has two oppositesurfaces along it thickness direction, and the crack 1311 penetrates thetwo surfaces.

It can be understood that, although the crack 1311 in FIG. 3 extendssubstantially uniformly along the length direction of the firstdielectric layer 131, in other embodiments, the direction of the crack1311 in the first dielectric layer 131 may be inconsistent. They can bein any direction.

In the negative electrode plate 10 provided by an embodiment of theapplication, due to the presence of the inorganic dielectric layer 13comprising the crack 1311 on the surface of the negative active materiallayer 12 facing away from the negative current collector 11 and on thesurface facing toward positive electrode, the inorganic dielectric layer13 can insulate electrolyte and negative active material. In addition,due to its dielectric property, the inorganic dielectric layer 13 canachieve the effect of stabilizing the negative electrode interface,thereby greatly reducing the side reaction between the electrolyte andthe negative electrode interface. Even under high temperature and fastcharging conditions, it can also effectively control the side reactionbetween the electrolyte and the negative electrode interface, inhibitgas production, and effectively suppress the increase of irreversiblecapacity, thereby improving the safety performance, cycle performanceand high temperature performance of electrochemical device. Inparticular, it results in an electrochemical device having high safetyperformance and cycle performance over a wide temperature range.

The inorganic dielectric layer 13 has a porous structure inside, forminga large number of ion migration channels. The inorganic dielectric layer13 has a large number of polar groups on the surface of the material,and has a good ion conductivity property. Further, the inorganicdielectric layer 13 has crack 1311, which provides open channels andincreases the contact surface area between electrolyte and electrodeplate, thereby stabilizing the negative electrode interface andmeanwhile increasing wetting and retention of the electrolyte on thenegative electrode plate, and improving ion permeability of negativeelectrode plate. In the case of charging and discharging at a highcurrent, it can also facilitate the intercalation and de-intercalationof electrolyte cations to ensure good kinetic performance ofelectrochemical device. Therefore, the application can effectivelycontrol the problem of the precipitation of the reduced metal of theelectrolyte cations on the surface of the negative electrode, avoidshort circuit in electrochemical device, and further improve safetyperformance and cycle performance. Moreover, it is advantageous inreducing the interface resistance and improving the utilization ofnegative active material, thereby improving capacity and rateperformance of the electrochemical device.

The inorganic dielectric layer 13 may also significantly inhibit theincrease of Direct Current Resistance (DCR) during cycle and storage ofelectrochemical device, thereby reducing thermal effects, reducingpolarization, and improving capacity and rate performance.

Moreover, since the inorganic dielectric layer 13 does not contain abinder, the inorganic dielectric layer 13 is not bonded to the negativeactive material layer 12 by a binder, so that safety performance, cycleperformance and high temperature performance of electrochemical devicecan be better improved. It may also prevent peeling off of the inorganicdielectric layer 13 from the surface of the negative active materiallayer 12, which may occur due to swelling and failure of binder duringcycle and storage process.

In addition, since the thickness of the inorganic dielectric layer 13 isnanoscale, the volume and weight of negative electrode plate are barelychanged, and the energy density of the electrochemical device is notlowered.

Therefore, the negative electrode plate provided by the application canprovide an electrochemical device with excellent overall electrochemicalperformance.

Further, the thickness T of the first dielectric layer 131 may have anupper limit of 1000 nm, 990 nm, 950 nm, 900 nm, 850 nm, 800 nm, 750 nm,720 nm, 700 nm, 680 nm, 650 nm, 600 nm, 550 nm, 500 nm, 490 nm, 450 nm,430 nm, 400 nm, 380 nm, 350 nm, 300 nm, 280 nm, 250 nm, 200 nm; thethickness T may have a lower limit of 30 nm, 35 nm, 40 nm, 45 nm, 50 nm,55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm,110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm.The thickness T of the first dielectric layer 131 may be any valuebetween the above upper and lower limits. The thickness T of the firstdielectric layer 131 is preferably from 50 nm to 600 nm, more preferablyfrom 100 nm to 500 nm, which may better exert the above effects of theinorganic dielectric layer 13 and may be advantageous for improving massenergy density and volume energy density of electrochemical device.

The ratio C_(r)/T of reversible capacity C_(r) per 1540 mm² area of thenegative electrode plate 10 to the thickness T of the first dielectriclayer 131 is preferably from 0.02 mAh/nm to 2 mAh/nm. This isadvantageous in ensuring that the above effects of the inorganicdielectric layer 13 can be effectively exhibited when the negativeelectrode plate 10 is made of various negative active materials, therebyimproving safety performance, cycle performance, and high temperatureperformance of electrochemical device.

Here, the reversible capacity C_(r) per 1540 mm² area of the negativeelectrode plate 10 means the reversible capacity that is possessed bythe negative electrode plate 10 per 1540 mm² area, and may be tested bythe following procedure: cutting the negative electrode plate of 1540mm² area, and preparing lithium metal piece having the same area as acounter electrode, assembling into a button type battery, and thenperforming the charging and discharging test to obtain the reversiblecapacity of the negative electrode plate of 1540 mm² area, i.e. thereversible capacity C_(r) per 1540 mm² area of the negative electrodeplate 10.

Preferably, the crack 1311 has a width of from 0.05 μm to 6 μm, andpreferably, a length-width ratio of 50 or more. This can ensure thetransmission performance of electrolyte cations while stabilizing thenegative electrode interface and suppressing the gas production, so thatthe negative electrode plate 10 has both excellent kinetic performanceand chemical stability. It may also improve the bonding between theinorganic dielectric layer 13 and the negative active material layer 12.Even when the negative active material layer 12 and the inorganicdielectric layer 13 are differently expand with varying degrees underconditions such as overcooling or overheating, the stress of theinorganic dielectric layer 13 can still be effectively released, therebypreventing peeling off of the inorganic dielectric layer 13 from thenegative active material layer 12 and improving stability. Further, thecrack 1311 has a width of from 0.5 μm to 6 μm, for example from 3 μm to6 μm.

The first dielectric layer 131 may have a coverage on the first surface121 of the negative active material layer 12 of from 70% to 95%, forexample from 80% to 90%, so that the negative electrode plate 10 hasboth high kinetic performance and stability.

Further, referring to FIGS. 4 to 7, the inorganic dielectric layer 13may also comprise second dielectric layer 132, disposed on inner wall ofat least a portion of pores inside the negative active material layer12, and the second dielectric layer extends on a direction from thefirst surface to the second surface of the negative active materiallayer 12. The second dielectric layer 132 has a porous structure inside.This is beneficial to improve the stability of the negative electrodeinterface and reduce the side reaction between electrolyte and negativeactive material, thereby effectively suppressing the gas production ofthe electrochemical device. This may also improve wetting and retentionof the electrolyte on the negative electrode plate 10, thereby furtherimproving safety performance and cycle performance of electrochemicaldevice, and improving the capacity and rate performance of theelectrochemical device.

Preferably, the ratio H/D of the depth H of the second dielectric layer132 extending on the direction from the first surface 121 to the secondsurface of the negative active material layer 12 to the thickness D ofthe negative active material layer 12 is from 1/1000 to 1/10. This isbeneficial to improve the stability of the negative electrode interface,improve the wetting and retention of the electrolyte on the negativeelectrode plate 10 and meanwhile ensure the electrical conductivities ofions and electrons, thereby improving electrochemical performance of theelectrochemical device.

In some embodiments, the second dielectric layer 132 has a thickness dof 0<d≤500 nm, for example 10 nm≤d≤500 nm, further for example 10nm≤d≤100 nm.

Preferably, the second dielectric layer 132 has a thickness d showing agradient decrease on a direction from the first surface 121 to thesecond surface 122. As a result, the inside of the negative electrodeplate 10 has a relatively large pore diameter and a relatively highporosity, and the pore diameter and the porosity gradually decrease fromthe inside to the outside. The pore diameter is small and the porosityis low at the first surface 121. This is advantageous for improving thewetting and retention of the electrolyte on the negative electrode plate10, thereby improving safety performance and cycle performance ofelectrochemical device.

Preferably, the negative electrode plate 10 of the application has acompact density of from 1.2 g/cm³ to 2.0 g/cm³, and a porosity ofpreferably from 25% to 55%. The negative electrode plate 10 has a stablenegative electrode interface and high ion and electron conductivities,so that the electrochemical device has high safety performance and cycleperformance.

The compact density C of negative electrode plate 10 may be calculatedby the equation C=(m×w)/(A_(n)×δ), wherein m is the mass of the negativeactive material layer, w is the mass fraction of the negative activematerial in the negative active material layer, A_(n) is the area of thenegative active material layer, and δ is the thickness of the negativeactive material layer.

The porosity of the negative electrode plate 10 may be measured bymercury intrusion method or gas adsorption method.

The inorganic dielectric material in the inorganic dielectric layer 13has an average particle diameter D_(v)50 of preferably from 1 nm to 10nm, which is favorable for forming a channel for ion migration betweenmaterial particles and improving ion conductivity property.

As an example, the above inorganic dielectric material may be one ormore of compounds comprising an element A and an element B, wherein theelement A may be one or more selected from the group consisting of Al,Si, Ti, Zn, Mg, Zr, Ca and Ba, and the element B may be one or moreselected from the group consisting of O, N, F, Cl, Br and I. Forexample, an inorganic dielectric material may be one or more selectedfrom the group consisting of oxide of aluminum, AlO(OH), nitride ofaluminum, fluoride of aluminum, oxide of silicon, nitride of silicon,fluoride of silicon, oxide of titanium, nitride of titanium, fluoride oftitanium, zinc oxide, zinc nitride, zinc fluoride, oxide of magnesium,nitride of magnesium, halide of magnesium, oxide of zirconium, nitrideof zirconium, fluoride of zirconium, oxide of calcium, nitride ofcalcium, fluoride of calcium, oxide of barium, nitride of barium andfluoride of barium.

The thickness of the anode active material layer 12 is not particularlylimited in the application, and may be adjusted by those skilled in theart according to actual needs. In some embodiments, the negative activematerial layer 12 may have a thickness of from 90 μm to 200 μm, forexample from 100 μm to 130 μm.

The negative active material is not particularly limited in theapplication, and may be a negative active material which is known forcapable of being intercalated by electrolyte cations in the art. In someembodiments, the negative active material may have an average particlediameter D_(v)50 of from 6 μm to 10 μm.

For example, in a negative electrode plate for a lithium ion battery,the negative active material may be one or more selected from the groupconsisting of natural graphite, artificial graphite, soft carbon, hardcarbon, silicon, silicon oxides, silicon carbon composites, Li—Sn alloy,Li—Sn—O alloy, Sn, SnO, SnO₂, Li₄Ti₅O₁₂ with spinel structure, Li—Alalloy and lithium metal. Wherein, silicon oxides refer to SiOx, x<2, forexample silicon monoxide and the like; silicon carbon composites may beone or more selected from the group consisting of carbon-coated silicon,carbon-coated silicon oxide, a mixture of silicon and carbon, a mixtureof silicon oxide and carbon, a mixture of silicon and silicon oxide andcarbon, wherein the carbon may be one or more selected from the groupconsisting of graphite, soft carbon, and hard carbon.

In the negative electrode plate 10 of the application, the negativeactive material layer 12 may also optionally comprises a binder and aconductive agent. The types of the binder and the conductive agent arenot particularly limited, and may be selected according to actualrequirements. As an example, the binder may be one or more selected fromthe group consisting of styrene-butadiene rubber (SBR), water-basedacrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride(PVDF), polyvinyl butyral (PVB), polytetrafluoroethylene (PTFE),ethylene-vinyl acetate copolymer (EVA), and polyvinyl alcohol (PVA). Theconductive agent may be one or more selected from the group consistingof graphite, superconducting carbon, acetylene black, One or more ofcarbon black, ketjen black, carbon dots, carbon nanotubes, graphene, andcarbon nanofibers.

In the negative electrode plate 10 of the application, the type of thenegative current collector 11 is not particularly limited and may beselected according to actual requirements. For example, a material suchas a metal foil, a carbon-coated metal foil or a porous metal plate maybe used as a negative current collector, such as a copper foil.

An embodiment of the application further provides a method for thepreparation of a negative electrode plate 10, comprising the step:

Step S100, disposing a negative active material layer on at least onesurface of a negative current collector.

Step S100 may be carried out by the any of the following ways:

Dispersing a negative active material and optionally a binder and aconductive agent in a solvent, for example deionized water orN-methylpyrrolidone (NMP), to form a uniform negative electrode slurry;coating or spraying the negative electrode slurry on a surface of thenegative current collector; and drying or the like, to form the negativeactive material layer on at least one surface of the negative currentcollector.

A negative active material layer may be obtained by direct preparing thenegative active material on the negative current collector. For example,the negative active material may be directly prepared on the negativecurrent collector by vapor deposition. Vapor deposition method may beone or more of Atomic Layer Deposition (ALD), Chemical Vapor Deposition(CVD), and Physical Vapor Deposition (PVD).

A lithium metal foil was prepared on the anode current collector toobtain a cathode active material layer. For example, a lithium metalfoil is pressed onto the negative current collector.

A negative active material layer may be obtained by preparing a thinlithium metal sheet on the negative current collector. For example, athin lithium metal sheet may be pressed on the negative currentcollector.

Step S200, depositing an inorganic dielectric material on a surface ofthe negative active material layer facing away from the negative currentcollector by vapor deposition, to obtain an initial inorganic dielectriclayer.

Vapor deposition may be one or more selected from the group consistingof Atomic Layer Deposition (ALD), Chemical Vapor Deposition (CVD), andPhysical Vapor Deposition (PVD). The physical vapor deposition method ispreferably one or more selected from the group consisting ofevaporation, sputtering, and Arc Ion Plating (AIP), for example, ThermalEvaporation Deposition, Plasma assisted Thermal Evaporation, ElectronBeam Evaporation Method (EBEM), Plasma assisted electron beamEvaporation, Reactive Ion-beam Sputtering (RIBS), Magnetron sputtering,and Arc Ion Plating (AIP).

During vapor deposition, as the inorganic dielectric material isdeposited on a surface of the negative active material layer to form aninitial first dielectric layer, a second dielectric layer issimultaneously formed on inner wall of pores on a surface of thenegative active material layer. As the thickness of the initial firstdielectric layer is gradually increased, the second dielectric layergradually extends toward the inside of the negative active materiallayer, and its thickness is gradually decreased from the outside to theinside.

The parameters such as composition, thickness and morphology of thefirst dielectric layer and the second dielectric layer may be modulatedby adjusting the porosity and average pore diameter of the negativeactive material layer, the average particle diameter of the anode activematerial, and adjusting one or more of processing parameters includingthe type and concentration of reaction raw materials for vapordeposition and reaction temperature, surface temperature of the negativeactive material layer, deposition distance, deposition rate, and thedeposition time, and the like.

In case the inorganic dielectric layer is prepared by thermalevaporation, step S200 may include the following steps:

Step S210, vacuum pumping a deposition chamber to a gas pressure of 0.1Pa or less, such as 0.001 Pa or less.

Step S220, introducing a reactive gas a into the deposition chamber. Thereactive gas a may be one or more selected from the group consisting ofoxygen, ozone, carbon dioxide, water vapor, nitrogen, nitrogen monoxide,nitrogen dioxide, and ammonia.

Step S230, heating an inorganic dielectric material precursor b into agaseous state in a heating chamber, and introducing it into thedeposition chamber. The inorganic dielectric material precursor b may beone or more selected from the group consisting of a simple substance, analloy, an alkyl compound, a nitrate, an acetate, and a sulfatecontaining the element A.

Step S240, reacting the reactive gas a with the precursor of the gaseousinorganic dielectric material by adjusting the concentration of theinorganic dielectric material precursor b in a gaseous state in thedeposition chamber, the temperature in the chamber, deposition distance,deposition time and the like, to form an initial inorganic dielectriclayer on a surface of the negative active material layer.

In case the inorganic dielectric layer is prepared by plasma assistedelectron beam evaporation, step S200 may include the following steps:

Step S210′, vacuum pumping a reaction chamber to a gas pressure of 0.1Pa or less, such as 0.001 Pa or less.

Step S220′, introducing the reactive gas a into the inductively coupledplasma (ICP) source. The reactive gas a may be diluted with an inertgas. Under the action of the ICP source, the reactive gas a generates aplasma containing element B.

Here, the reactive gas a is as described above. The inert gas may be,for example, one or more selected from the group consisting of argongas, nitrogen gas, and helium gas.

The ICP source may have a power of from 300 W to 600 W. The reactive gasa may have a flow rate of from 200 sccm to 500 sccm.

Step S230′, in the reaction chamber, bombarding a target c containingelement A with an electron beam generated by an electron gun, to meltand evaporate the target c; chemically reacting the resulting gaseousmaterial with the plasma and depositing it on a surface of the negativeactive material layer to form an initial inorganic dielectric layer.

Herein, the target c containing element A may be one or more selectedfrom the group consisting of a simple substance and an alloy containingelement A.

The electron beam may have a voltage of from 6 kV to 12 kV. Furthermore,surface temperature of the negative active material layer is preferablycontrolled in a range of 100° C. or less.

The parameters such as composition of the initial inorganic dielectriclayer, and its thickness, morphology and the like on the negative activematerial layer may be modulated by adjusting one or more of theparameters including vacuum degree in the reaction chamber, voltage ofthe electron beam, composition and flow rate of the reactive gas a,composition of the target c containing element A, ICP source, surfacetemperature of the negative active material layer, and the process time.

Step S300, applying pressure to the initial first dielectric layer toform a crack in the initial first dielectric layer, to obtain a firstdielectric layer as an inorganic dielectric layer, yielding the negativeelectrode plate.

In Step S300, a surface of the product obtained in step S200 may becold-pressed by using known devices and methods for cold-pressingtreatment. After the cold-pressing treatment, the first dielectric layerforms a discontinuous lamellar structure having cracks, so that thenegative electrode plate has more ion channels and larger surface area,which is beneficial to ensure good kinetic performance and rateperformance of electrochemical device.

According to the method for the preparation of the negative electrodeplate 10 described in the embodiment of the application, theabove-described negative electrode plate 10 of the application beobtained, so that the resulting electrochemical device has excellentsafety performance and cycle performance, and meanwhile has the improvedkinetic performance, rate performance and high temperature performanceof electrochemical device.

Electrochemical Device

The second aspect of the embodiment of the application provides anelectrochemical device. The electrochemical device may be, but notlimited thereto, a lithium ion secondary battery, a lithium primarybattery, a sodium ion battery, or a magnesium ion battery.

The electrochemical device comprises a positive electrode plate, anegative electrode plate, a separator, and an electrolyte, wherein thenegative electrode plate is the negative electrode plate provided by thefirst aspect of the embodiment of the application.

By using the negative electrode plate of the first aspect of theembodiment of the application, the electrochemical device has excellentsafety performance and cycle performance, and meanwhile has the improvedrate performance and high temperature performance of the electrochemicaldevice.

Hereinafter, an electrochemical device will be further described bytaking a lithium ion secondary battery as an example.

In a lithium ion secondary battery, a positive electrode plate maycomprise a positive current collector and a positive active materiallayer disposed on at least one surface of the positive currentcollector.

The positive current collector may be a metal foil, a carbon-coatedmetal foil or a porous metal plate, for example an aluminum foil.

The thickness of the positive active material layer is not particularlylimited in the application and may be adjusted by those skilled in theart according to actual needs. In some embodiments, the positive activematerial layer may have a thickness of from 100 μm to 180 μm, forexample from 110 μm to 130 μm.

The positive active material in the positive active material layer isnot particularly limited, as long as it is a compound capable ofreversibly intercalating/de-intercalating lithium ions. For example, thepositive active material may be a lithium transition metal compositeoxide, wherein the transition metal may be one or more selected from thegroup consisting of Mn, Fe, Ni, Co, Cr, Ti, Zn, V, Al, Zr, Ce and Mg.The lithium transition metal composite oxide may be, for example, one ormore selected from the group consisting of LiMn₂O₄, LiNiO₂, LiCoO₂,LiNi_(1-y)Co_(y)O₂ (0<y<1), LiNi_(a)Co_(b)Al_(1-a-b)O₂ (0<a<1, 0<b<1,0<a+b<1), LiMn_(1-m-n)Ni_(m)Co_(n)O₂ (0<m<1, 0<n<1, 0<m+n<1), LiMPO₄ (Mmay be one or more of Fe, Mn, and Co) and Li₃V₂(PO₄)₃. In the lithiumtransition metal composite oxide, an element having a largeelectronegativity, such as one or more of S, N, F, Br, Cl, and I, mayalso be doped. The lithium transition metal composite oxide may also besubjected to a coating modification treatment. The doping and/or coatingmodification enables the compound to have more stable structure andbetter electrochemical performance.

The positive active material layer may comprise a binder and aconductive agent. The above binder is not particularly limited, and maybe for example one or more selected from the group consisting ofstyrene-butadiene rubber (SBR), water-based acrylic resin, carboxymethylcellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene(PTFE), polyvinyl butyral (PVB), ethylene-vinyl acetate copolymer (EVA),and polyvinyl alcohol (PVA). The above conductive agent is notparticularly limited, and may be for example one or more selected fromthe group consisting of graphite, superconducting carbon, acetyleneblack, carbon black, ketjen black, carbon dots, carbon nanotubes,graphene, and carbon nanofibers.

The positive electrode plate may be prepared by conventional method inthe art. Usually, a positive electrode active material and a conductiveagent and a binder are dispersed in a solvent (for example,N-methylpyrrolidone, abbreviated as NMP), to form a uniform positiveelectrode slurry. The positive electrode slurry is coated on thepositive current collector. After the steps of drying, cold pressing,etc., the positive electrode plate is obtained.

The separator in lithium ion secondary battery is not particularlylimited, and be any well-known porous structural separator havingelectrochemical stability and chemical stability. For example, a singleor multilayer film of one or more selected from the group consisting ofglass fiber, nonwoven fabric, polyethylene, polypropylene, andpolyvinylidene fluoride may be used.

The electrolyte of lithium ion secondary battery comprises an organicsolvent and an electrolyte lithium salt.

The above organic solvent is not particularly limited, and may beselected according to actual requirements. For example, the organicsolvent may be one or more, preferably two or more, selected from thegroup consisting of ethylene carbonate (EC), propylene carbonate (PC),ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethylcarbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate(MPC), ethylpropyl carbonate (EPC), butylene carbonate (BC), fluorinatedethyl carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethylacetate (EA), propyl acetate (PA), methyl propionate (MP), ethylpropionate (EP), propyl propionate (PP), methyl butyrate (MB), ethylbutyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone(MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

The type of the above electrolyte lithium salt is not particularlylimited, and may be selected according to actual requirements. Forexample, the electrolyte lithium salt may be one or more selected fromthe group consisting of LiPF₆ (lithium hexafluorophosphate), LiBF₄(lithium tetrafluoroborate), LiClO₄ (lithium perchlorate), LiAsF₆(lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide),LiTFSI (lithium bis(trifluoromethanesulphonyl)imide), LiTFS (lithiumtrifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate),LiBOB (lithium bis(oxalate)borate), LiPO₂F₂ (lithium difluorophosphate),LiDFOP (lithium difluorodioxalate phosphate) and LiTFOP (lithiumtetrafluorooxalate phosphate). Preferably, the electrolyte lithium saltin electrolyte has a concentration of from 0.5 mol/L to 1.5 mol/L,further preferably from 0.8 mol/L to 1.2 mol/L.

The electrolyte may further optionally comprise additives. The type ofthe additive is not particularly limited and may be selected accordingto actual requirements. For example, the additives may be one or moreselected from the group consisting of vinylene carbonate (VC), vinylethylene carbonate (VEC), fluoroethylene carbonate (FEC), succinonitrile(SN), adiponitrile (ADN), 1,3-propene sultone (PST),tris(trimethylsilyl)phosphate (TMSP) and tris(trimethylsilyl)borate(TMSB).

The above electrolyte may be prepared in accordance with a conventionalmethod in the art. An electrolyte may be obtained by uniformly mixing anorganic solvent and an electrolyte lithium salt and optional additive.Here, the sequence in which the materials are added is not particularlylimited. For example, an electrolyte may be obtained by adding anelectrolyte lithium salt and optional additive into an organic solventand uniformly mixing. Here, the electrolyte lithium salt may be firstadded to the organic solvent, and then the optional additive may beseparately or simultaneously added to the organic solvent.

The above positive electrode plate, separator and negative electrodeplate are stacked in order, so that the separator is placed between thepositive electrode plate and the negative electrode plate to take afunction of isolating, thereby obtaining a battery core. A battery coremay also be obtained after winding. The battery core is placed in apackage. After injection of an electrolyte, the package is sealed toobtain a lithium ion secondary battery.

EXAMPLES

The present disclosure is more particularly described in the followingexamples that are intended as illustrations only, since numerousmodifications and variations within the scope of the present disclosurewill be apparent to those skilled in the art. Unless otherwise noted,all parts, percentages, and ratios reported in the following examplesare on a weight basis, and all reagents used in the examples arecommercially available and used directly without further treatment, andthe instruments used in the examples are commercially available.

Example 1

Preparation of Negative Electrode Plate

Step S110, mixing and adding a negative active material, a conductiveagent acetylene black, a binder styrene-butadiene rubber (SBR), athickener sodium carboxymethyl cellulose (CMC) into a solvent deionizedwater at a mass ratio of 96:1:2:1. After stirring and mixing uniformly,a negative electrode slurry was obtained. Here, the negative activematerial was artificial graphite.

Step S120, uniformly coating the above negative electrode slurry on bothsurfaces of a negative current collector copper foil, and then drying at80° C. to 90° C., to obtain a negative active material coating.

Step S200, depositing an inorganic dielectric material on a surface ofthe negative active material coating facing away from the negativecurrent collector by using a plasma-assisted electron beam evaporationmethod, to obtain an initial negative electrode plate. This stepspecifically included:

Step S210′, vacuum pumping a reaction chamber to a gas pressure of 0.001Pa or less.

Step S220′, introducing oxygen into an ICP source, to generate oxygenplasma under the action of the ICP source. Here, the ICP source had apower of 300 W. The flow rate of oxygen was 300 sccm.

Step S230′, in the reaction chamber, bombarding an aluminum target cwith an electron beam, so that it was heated to from 600° C. to 650° C.to melt and evaporate. Aluminum vapor chemically reacted with oxygenplasma and were deposited on a surface of the negative active materialcoating, wherein the temperature of the negative active material coatingwas controlled in a range of 100° C. or less, to form an initialinorganic dielectric layer. A negative electrode plate was formed.

Step S300, further subjecting the initial negative electrode plate tocold pressing, trimming, cutting, slitting, and finally drying undervacuum condition at 85° C. for 4 hours, thereby obtaining a negativeelectrode plate. The negative active material layer had a thickness of101 μm. The first dielectric layer had a discontinuous lamellarstructure having cracks and a thickness T of 30 nm.

Preparation of Positive Electrode Plate

A ternary material LiNi_(0.8)Co_(0.0)Mn_(0.1)O₂, conductive carbon black(SP), and a binder polyvinylidene fluoride (PVDF) were mixed at a weightratio of 96:2:2 and added into a solvent N-methylpyrrolidone (NMP).After thoroughly stirring and mixing, a uniform positive electrodeslurry was formed. The positive electrode slurry was coated on apositive current collector aluminum foil, and then dried at 85° C.,followed by cold pressing, trimming, cutting, and slitting, and thendried under vacuum at 85° C. for 4 h to obtain a positive electrodeplate. The thickness of positive active material layer in the positiveelectrode plate was 119 μm.

Preparation of Electrolyte

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethylcarbonate (DEC) were uniformly mixed at a volume ratio of 20:20:60 toobtain an organic solvent. In an argon atmosphere glove box having awater content of less than 10 ppm, 1 mol/L of LiPF₆ was dissolved in theabove organic solvent and uniformly mixed to obtain an electrolyte.

Preparation of Lithium Ion Secondary Battery

The positive electrode plate, the separator, and the negative electrodeplate were stacked in order. A polyethylene (PE) film with a thicknessof 14 μm was used as a separator, and placed between the positiveelectrode plate and the negative electrode plate to take a function ofisolating. Then the stack was wound into a bare battery core. Afterwelding, the bare battery core was packed into aluminum foil casing,followed by baking at 80° C. to remove water. After injection ofelectrolyte, the package was sealed. A flexible lithium ion secondarybattery was obtained after standing, chemical treatment (charged to 3.3V with a constant current of 0.02 C and then charged to 3.6 V with aconstant current of 0.1 C), shaping, capacity test, and the like. Theflexible lithium ion secondary battery had a thickness of 4.0 mm and awidth of 60 mm and a length of 140 mm.

Examples 2 to 14

Different from Example 1, some relevant parameters in the preparationsteps of the negative electrode plate were adjusted.

Comparative Examples 1 to 4

Different from Example 1, no inorganic dielectric layer was provided andstep S200 was omitted and some relevant parameters in the preparationsteps of the negative electrode plate were adjusted.

Comparative Examples 5 to 6

Different from Example 1, some relevant parameters in the preparationsteps of the negative electrode plate were adjusted.

Comparative Example 7

Different from Example 1, step 120 included uniformly coating thenegative electrode slurry on both surfaces of a negative currentcollector copper foil, followed by drying at a temperature of from 80°C. to 90° C. and cold pressing, to obtain a negative active materiallayer; no cold pressing was performed in step S300; the inorganicdielectric layer has a continuous lamellar structure; and some relevantparameters in the preparation steps of the negative electrode plate wereadjusted.

Comparative Example 8

Different from Example 1, in step S200, an inorganic dielectric layerwas prepared on a surface of a negative active material coating by acoating method; and some relevant parameters in the preparation steps ofthe negative electrode plate were adjusted.

Step S200 specifically included: mixing alumina particles with a binderto obtain a mixed slurry, wherein the binder was hydroxymethyl cellulose(CMC) present in an amount of 30% by weight; Applying the mixed slurryto a surface of the negative active material layer facing away from thenegative current collector, followed by drying, to obtain an inorganicdielectric layer having a continuous lamellar structure.

The relevant parameters of Examples 1 to 14 and Comparative Examples 1to 8 were detailed in Table 1 below.

Tests

(1) Test on Gas Production During High-Temperature Storage of LithiumIon Secondary Battery

At 25° C., each of the lithium ion secondary batteries prepared inExamples and Comparative Examples were charged at a constant current of0.5 C to a voltage of 4.2 V, and then charged at a constant voltage of4.2 V until the current was 0.05 C. At this moment, the initial volumeof the secondary battery was measured and recorded as V₀. Then, thelithium ion secondary battery was placed in a thermostatic oven at 80°C. for 360 hours. After the completion of storage, the battery was takenout. The volume of the lithium ion secondary battery was measured andrecorded as V₁. For each group, 15 lithium ion secondary batteries weretested and the values were averaged. In this test, the volume of thelithium ion secondary battery was tested using the water displacementmethod.

The volume expansion ratio (%) of the lithium ion secondary batteryafter storage at 80° C. for 360 hours=(V₁−V₀)/V₀×100%

(2) Test on Cycle Performance at High Temperature of Lithium IonSecondary Battery

At 45° C., each of the lithium ion secondary batteries prepared inExamples and Comparative Examples were charged at a constant current of1 C to a voltage of 4.2 V, and charged at a constant voltage of 4.2 Vuntil the current was 0.05 C, and then discharged at a constant currentof 1 C to a voltage of 2.8 V. This was a charge/discharge cycle. Thedischarge capacity of at this time was the discharge capacity at thefirst cycle. The lithium ion secondary battery was subjected to 1000charge/discharge cycles in accordance with the above method. Dischargecapacity in each cycle was recorded. For each group, 15 lithium ionsecondary batteries were tested and the values were averaged.

The capacity retention ratio (%) of the lithium ion secondary batteryafter 1000 1C/1C cycles at 45° C.=discharge capacity at the 1000thcycle/discharge capacity at the first cycle×100%.

(3) Test on Storage Performance at High Temperature of Lithium IonSecondary Battery

At 25° C., each of the lithium ion secondary batteries prepared inExamples and Comparative Examples were charged at a constant current of0.5 C to a voltage of 4.2 V, and charged at a constant voltage of 4.2 Vuntil the current was 0.05 C, and then discharged at a constant currentof 1 C to a voltage of 2.8 V. The initial discharge capacity C₀ wasobtained. The lithium ion secondary battery was charged at a constantcurrent of 0.5C to a voltage of 4.2V, and then charged at a constantvoltage of 4.2V until the current was 0.05 C. The fully charged lithiumion secondary battery was placed in a thermostatic oven at 60° C. for180 days. After being taking out of thermostatic oven, the reversiblecapacity of the lithium ion secondary battery was measured, and recordedas C₁₈₀. For each group, 15 lithium ion secondary batteries were testedand the values were averaged.

The capacity retention ratio (%) of the lithium ion secondary batteryafter storage at 60° C. for 180 days=C₁₈₀/C₀×100%.

(4) Test on Direct Current Resistance (DCR) Growth of Lithium IonSecondary Battery after 1000 Cycles at High Temperature and StoragePerformance at High Temperature for 180 Days

At 25° C., first, each of fresh lithium ion secondary batteries preparedin Examples and Comparative Examples were charged at a constant currentof 1 C to a voltage of 4.2 V, and charged at a constant voltage of 4.2 Vuntil the current was 0.05 C, and then discharged at a constant currentof 1 C to a voltage of 2.8 V. The actual discharge capacity wasrecorded. With the actual discharge capacity, the state of charge (SOC)of the lithium ion secondary battery was adjusted to 20% of the fullcharge capacity. After the completion of the adjustment, the voltage ofthe lithium ion secondary battery at this moment was measured andrecorded as U₁. Then the lithium ion secondary battery was discharged ata rate of 0.3 C for 10 s. The voltage after discharge of the lithium ionsecondary battery was measured and recorded as U₂.

The initial Direct Current Resistance DCR₀ of lithium ion secondarybattery=(U₁-U₂)/I.

The Direct Current Resistance DCR₁ of the lithium ion secondary batteryafter 1000 cycles at 45° C. was tested according to the above method.The DCR growth of the lithium ion secondary battery was calculated. Foreach group, 15 lithium ion secondary batteries were tested and thevalues were averaged.

DCR growth (%) of the lithium ion secondary battery after 1000 1C/1Ccycles at 45° C.=(DCR₁-DCR₀)/DCR₀×100%.

Similarly, the Direct Current Resistance DCR₁₈₀ of the lithium ionsecondary battery after storage at 60° C. for 180 days was tested inaccordance with the above method. The DCR growth of the lithium ionsecondary battery was calculated. For each group, 15 lithium ionsecondary batteries were tested and the values were averaged.

The DCR growth (%) of the lithium ion secondary battery after storage at60° C. for 180 days=(DCR₁₈₀−DCR₀)/DCR₀×100%.

(5) Test on Lithium Precipitation on Surface of Negative Electrode UnderHigh-Current Charging of Lithium Ion Secondary Battery

At 25° C., first, each of fresh lithium ion secondary batteries preparedin Examples and Comparative Examples were charged at a constant currentof 2 C to a voltage of 4.2 V, and charged at a constant voltage of 4.2 Vuntil the current was 0.05 C, and then discharged at a constant currentof 2 C to a voltage of 2.8 V. This was a charge/discharge cycle. Thelithium ion secondary battery was subjected to 10 high-currentcharge/discharge cycles, and then charged at a constant current of 2 Cto a voltage of 4.2 V, and charged at a constant voltage of 4.2 V untilthe current was 0.05 C.

The battery was fully charged and disassembled. The negative electrodeplate was taken. The precipitation of lithium on the negative electrodeplate was observed and classified according to the following scale: A,no lithium precipitation; B, slight lithium precipitation; C, alocalized region formed by lithium; D, lithium residue in most regions;E, severe lithium precipitation.

The test results of Examples 1 to 14 and Comparative Examples 1 to 8were shown in Table 2 below.

TABLE 1 Inorganic dielectric layer Negative electrode plate InorganicCompact Negative active dielectric T Morphology of first Width ofLength-width density C_(r)/T material material nm dielectric layer crackμm ratio of crack Porosity % g/cm³ mAh/nm Example 1 artificial graphitealumina 30 discontinuous layer 3 55 40 1.7 1.33 Example 2 artificialgraphite alumina 50 discontinuous layer 3 55 39 1.7 0.80 Example 3artificial graphite alumina 100 discontinuous layer 3 55 39 1.7 0.40Example 4 artificial graphite alumina 200 discontinuous layer 3 55 381.7 0.20 Example 5 artificial graphite alumina 500 discontinuous layer 460 37 1.7 0.08 Example 6 artificial graphite alumina 600 discontinuouslayer 5 62 37 1.7 0.07 Example 7 artificial graphite alumina 1000discontinuous layer 6 63 35 1.7 0.04 Example 8 artificial graphitealumina 200 discontinuous layer 3 55 55 1.2 0.20 Example 9 artificialgraphite alumina 200 discontinuous layer 3 55 50 1.4 0.20 Example 10artificial graphite alumina + silicon 200 discontinuous layer 3 55 511.7 0.20 oxide Example 11 artificial graphite alumina + silicon 200discontinuous layer 3 55 51 1.7 0.20 nitride Example 12 artificialgraphite + alumina 200 discontinuous layer 3 55 50 1.7 0.20 naturalgraphite Example 13 silicon oxide + alumina 200 discontinuous layer 3 5551 1.7 0.30 artificial graphite Example 14 silicon oxide alumina 200discontinuous layer 3 55 52 1.7 0.40 Comparative artificial graphite / // / / 40 1.7 / Example 1 Comparative artificial graphite + / / / / / 431.7 / Example 2 natural graphite Comparative silicon oxide + / / / / /44 1.7 / Example 3 artificial graphite Comparative silicon oxide / / / // 42 1.7 / Example 4 Comparative artificial graphite alumina 20discontinuous layer 3 55 32 1.7 2.00 Example 5 Comparative artificialgraphite alumina 1200 discontinuous layer 6 64 33 1.7 0.03 Example 6Comparative artificial graphite alumina 1200 continuous layer / / 30 1.70.03 Example 7 Comparative artificial graphite alumina 2000 continuouslayer / / 32 1.7 0.02 Example 8

TABLE 2 DCR growth Lithium precipitation on Volume expansion Capacityretention DCR growth after Capacity retention ratio after storage atsurface of negative ratio after storage at ratio after 1000 1000 cyclesat after storage at 60° C. for 60° C. for 180 electrode under high- 80°C. for 360 hours/% cycles at 45° C./% 45° C./% 180 days/% days/% currentcharging Example 1 23.5 82.6 33.4 85.6 24.1 A Example 2 19.6 83.2 24.886.1 26.4 A Example 3 18.7 86.3 23.0 87.4 23.7 A Example 4 20.3 87.722.6 86.1 23.3 A Example 5 20.8 87.9 22.3 86.5 22.9 B Example 6 19.982.3 22.6 84.2 22.1 B Example 7 21.7 82.5 23.7 85.6 23.4 C Example 822.5 82.0 25.4 83.2 24.4 A Example 9 22.0 83.6 23.3 85.4 25.4 A Example10 18.7 84.4 25.9 84.6 24.2 A Example 11 19.4 83.1 24.5 85.1 23.3 AExample 12 20.9 84.2 23.1 88.2 24.7 A Example 13 23.7 83.3 25.2 85.025.0 A Example 14 39.6 83.9 23.7 86.3 24.4 A Comparative 30.8 75.3 34.976.0 38.2 A Example 1 Comparative 29.1 75.3 36.1 77.9 36.3 B Example 2Comparative 32.6 74.4 37.0 74.5 36.0 B Example 3 Comparative 46.5 70.354.3 72.9 49.4 B Example 4 Comparative 27.7 78.5 34.8 77.4 30.1 BExample 5 Comparative 28.4 80.2 31.2 79.3 35.6 C Example 6 Comparative34.5 75.4 32.3 79.4 35.6 D Example 7 Comparative 30.4 74.5 33.1 75.432.1 E Example 8

From the comparisons of test results between Comparative Examples 1 to11 and Comparative Example 1, Example 12 and Comparative Example 2,Example 13 and Comparative Example 3, as well as Example 14 andComparative Example 4, It can be seen that: By disposing the aboveinorganic dielectric layer having crack on a surface of negative activematerial layer facing away from the negative current collector, lithiumion secondary battery after storage at 80° C. for 360 hours had asignificantly reduced volume expansion ratio, and gas production at hightemperature was effectively suppressed; the lithium precipitation onsurface of negative electrode under high-current charging of the lithiumion secondary battery was also effectively suppressed, thereby improvingsafety performance of battery; the capacity retention ratio (%) oflithium ion secondary battery after 100 1C/1C cycles at 45° C. wassignificantly increased; the Direct Current Resistance growth of lithiumion secondary battery after 100 1C/1C cycles at 45° C. was effectivelysuppressed, thereby improving cycle performance of battery; the cycleperformance at high temperature was also improved; the capacityretention ratio of lithium ion secondary battery after storage at 60° C.for 180 days was significantly improved; and the Direct CurrentResistance growth of lithium ion secondary battery after storage at 60°C. for 180 days was effectively suppressed, thereby improving thestorage performance at high temperature of battery.

From the comparisons of Examples 1 to 11 and Comparative Examples 5 to6, it can be seen that by using a first dielectric layer having athickness in a predetermined range on the outer surface of the negativeactive material layer, the effects of the inorganic dielectric layerwere effectively exerted, meanwhile battery had the improved safetyperformance, cycle performance and storage performance at hightemperature.

From the comparisons of Comparative Example 6 and Comparative Example 7,it can be seen that if the first dielectric layer on outer surface ofthe negative active material layer was a continuous layer, thesuppression on gas production and cycle performance of battery wereaffected to some extent, especially lithium precipitation on surface ofnegative electrode was aggravated under high current charging.

From the comparisons of Example 1 to 11 and Comparative Example 8, sincethe inorganic dielectric layer was prepared by a coating method inComparative Example 8, the inorganic dielectric layer was a continuousfilm layer bonded to outer surface of negative active material layer,the suppression on gas production, cycle performance and safetyperformance of battery were poor, and severe lithium precipitationoccurred on surface of negative electrode under high current charging.Since the inorganic dielectric layer was bonded to negative activematerial layer through a binder, the inorganic dielectric layer tendedto be peeled off due to swelling failure of the binder during cycle andstorage processes. It was also generally difficult to obtain aninorganic dielectric layer having a thickness of less than 2 μm bycoating method, and energy density of the resulting battery may also beaffected.

The above mentioned descriptions only show particular implementations ofthe present invention and but are not intended to limit the protectionscope of the present invention. Any modification or replacement readilyfigured out by a person skilled in the art within the technical scopedisclosed in the present invention shall fall within the protectionscope of the present invention. Therefore, the protection scope of thepresent invention shall be subject to the protection scope of theclaims.

1. A negative electrode plate, comprising: a negative current collector;a negative active material layer, disposed on at least one surface ofthe negative current collector, said negative active material layercomprises opposite first surface and second surface, wherein said firstsurface disposed away from the negative current collector; and aninorganic dielectric layer, disposed on the first surface of thenegative active material layer, said inorganic dielectric layerconsisting of an inorganic dielectric material, wherein said inorganicdielectric layer comprises a first dielectric layer on an outer surfaceof the negative active material layer, and the first dielectric layerhas a thickness of from 30 nm to 1000 nm and has a crack extendingthrough the thickness of the first dielectric layer.
 2. The negativeelectrode plate according to claim 1, wherein the first dielectric layerhas a thickness of from 50 nm to 600 nm.
 3. The negative electrode plateaccording to claim 1, wherein the first dielectric layer has a thicknessof from 100 nm to 500 nm.
 4. The negative electrode plate according toclaim 1, wherein a ratio of reversible capacity per 1540 mm² area of thenegative electrode plate to the thickness of the first dielectric layeris from 0.02 mAh/nm to 2 mAh/nm.
 5. The negative electrode plateaccording to claim 1, wherein the crack has a width of 0.05 μm to 6 μm,and a length-width ratio of 50 or more.
 6. The negative electrode plateaccording to claim 1, wherein the first dielectric layer has a coverageon the first surface of the negative active material layer of from 70%to 95%.
 7. The negative electrode plate according to claim 1, whereinthe inorganic dielectric layer further comprises second dielectric layeron inner wall of at least a portion of pores inside the negative activematerial layer, and the second dielectric layer extends on a directionfrom the first surface to the second surface.
 8. The negative electrodeplate according to claim 7, wherein a depth of the second dielectriclayer extending on the direction from the first surface to the secondsurface is from 1/1000 to 1/10 of the thickness of the negative activematerial layer.
 9. The negative electrode plate according to claim 7,wherein the second dielectric layer has a thickness d of 0<d≤500 nm;and/or the second dielectric layer has a thickness d showing a gradientdecrease on a direction from the first surface to the second surface.10. The negative electrode plate according to claim 1, wherein thenegative electrode plate has a compact density of from 1.2 g/cm³ to 2.0g/cm³ and a porosity of from 25% to 55%.
 11. The negative electrodeplate according to claim 1, wherein the inorganic dielectric materialcomprises one or more of compounds comprising an element A and anelement B, wherein the element A is one or more selected from the groupconsisting of Al, Si, Ti, Zn, Mg, Zr, Ca and Ba, and the element B isone or more selected from the group consisting of O, N, F, Cl, Br and I;and/or, said inorganic dielectric material has an average particlediameter D_(v)50 of from 1 nm to 10 nm.
 12. A method for preparation ofa negative electrode plate, comprising the steps of: disposing anegative active material layer on at least one surface of a negativecurrent collector; depositing an inorganic dielectric material on asurface of the negative active material layer facing away from thenegative current collector by vapor deposition, to obtain an initialinorganic dielectric layer including an initial first dielectric layeron an outer surface of the negative active material layer; applyingpressure to the initial first dielectric layer to form a crack extendingthrough the thickness of the initial first dielectric layer to obtain afirst dielectric layer as an inorganic dielectric layer, wherein thefirst dielectric layer has a thickness of from 30 nm to 1000 nm.
 13. Anelectrochemical device, comprising a positive electrode plate, anegative electrode plate, a separator, and an electrolyte, wherein thenegative electrode plate is the negative electrode plate according toclaim 1.